1. Field of the Invention
The invention relates to shaped structural members, particularly as-continuously cast beam blanks, from which finished structural beams are subsequently fashioned.
2. Description of the Related Art, Including Information Disclosed Under 37 C.F.R. .sctn..sctn. 1.97-1.99
Shaped structural members formed of metal, particularly of carbon or low-alloy steel, are used in various applications. Shaped structural members of various configurations are well-known to the metal forming art, and include beams. Beams conventionally have a web portion with opposed flanges extending from both ends of the web portion in a direction substantially normal thereto. Beams are usually formed from a casting of the steel, such as an ingot casting, which is subsequently hot worked by known methods to the desired finally-dimensioned and configured beam structure. Alternately, beams may be formed by a continuous casting operation which forms either a billet for subsequent hot working to form the beam or produces a shaped cross-section casting having a cross-section approximating the final configuration of the beam, which casting is then subjected to a series of hot and then cold rolling operations to form the finally dimensioned and configured beam product. Continuous casting has the advantage that a series of beam blanks may be formed from one or more heats of steel in a substantially continuous operation. This enables energy savings to be achieved and also improves the quantity of production. In the steel industry, the term "beam blank" denotes such a shaped cross section casting, a semifinished product with a shaped cross section approximating a beam configuration, which when subjected to further rolling steps is converted from that semifinished, as-cast state to a finished product having the desired and required final dimensions and specific, final configuration. Beam blanks are used as a precursor or starting material for the production of a variety of final structural member shapes, including H shaped beams, I shaped beams (usually referred to as "I beams") wide flange profile beams, British standard profile beams, Japanese industrial standard profile beams, and rail profiles, including railroad, crane and gantry rails.
As is well-known in the steel making art, hot rolling operations take the approximate-shape blank and reduce the shape to the finally dimensioned and shaped article, while altering the initial metallurgy and crystallization of the steel to the ultimate, desired state, with the required crystal state and form. Additional operations are then normally utilized to straighten the finally-dimensioned and configured member, and to cut the member to the desired length.
A mold for the continuous casting of such beam blanks typically has a central casting passage which is bounded by a pair of parallel walls which is designed to form the web of the beam blank. On either side of the central casting passage are second casting passages which each widen in a direction away from the central casting passage. These second or expanding casting passages are designed to form the inner portion of the flanges or flange precursors of the beam blank. Each of the expanding casting passages merges into a generally rectangular terminal casting passage designed to form the outer portion of the flanges or flange precursors of the beam blank.
Early attempts at shaped cross section casting, specifically including beam blanks, were first reported in about 1961 (N. N. Guglin, A. K. Provorny, G. F. Zasetskey, and B. B. Gulyaev, Stal (1961)), involving, on a laboratory scale, a simple 125.degree. wide angled section with two legs of unequal (30 and 40 mm, respectively) thickness. The casting encompassed an area of approximately 127 cm.sup.2. These laboratory scale experiments did not initially indicate the viability of the concept for use in continuous casting processes.
Certain other laboratory work was later carried out by British Iron and Steel Research Association ("BISRA") at its Sheffield Laboratories (H. S. Marr., B. Witt, B. W. H. Marsden, and R. I. Marshall, Journal of the Iron and Steel Institute, December 1966), to produce shaped cross section castings, including beam blanks. G.B. 1,049,698 (1965) describes symmetrical and asymmetrical shapes, including approximate configurations which could generally be described as roughly railroad rail-type in cross section, hour-glass type in cross section and I beam-type in cross section. The I beam-type cross section castings averaged 670 cm.sup.2 in area, with dimensions of 464 .times.254.times.76 (web length.times.flange height.times.web thickness, mm [181/4".times.10".times.3"]).
Further research activity undertaken by BISRA with Algoma Steel Corporation, Ltd. (Sault-Sainte-Marie, Ontario, Canada), studied the possibility of casting beam blanks for subsequent rolling to wide-flange universal I beams using the techniques described in G.B. 1,049,698. A commercial two (2) strand unit for continuous casting of such beam blanks was installed at Algoma in 1968. The beam blank sections cast by this installation averaged between 845-1435 cm.sup.2 in area, with dimensions of various combinations, including 451.times.305.times.102; 559.times.267.times.102; 775.times.356.times.102; 673 .times.260 .times.102; and 1164 .times.356 .times.102, mostly having the approximate I beam-type cross section.
A number of shaped cross section continuous casting devices for the production, inter alia, of beam blanks were installed in the period subsequent to 1968, which produced one or more of the three noted type cross section blanks. These comprised a number of Japanese installations, including those at Kawasaki Steel Corporation, a four (4) strand bloom/beam blank caster, installed at Mizushima, Okayawa, Japan (beam blank sections averaged 1155 cm.sup.2, with dimensions of 460 .times.400 .times.120 and 560 .times.287 .times.120); Tokyo Steel Manufacturing Co. Ltd's. single (1) strand unit at Kohchi Works, Shikoku, Japan (beam blank sections averaged 820 cm.sup.2, with dimensions of 445 .times.280 .times.110); a single (1) strand unit at the Himeji Works of Yamato Kogyo KK, Himeji, Japan (beam blank sections averaged 1100 cm.sup.2, with dimensions of 460 .times.370 .times.140); and a four (4) strand beam blank installation at Nippon Kohan KK's Fukuyama facility, Fukuyama, Japan (beam blank sections averaged 1145-1165 cm.sup.2, with dimensions of 480.times.400.times.120), as well as a number of European and Russian installations, including those at Mannesmann AG, Huttenwerke, Huckingen-Duisburg, West Germany (beam blank sections averaged 460 cm in area, with dimensions of 350 .times.210 .times.80); Research Development Works, Tula, USSR, described in O. V. Martynov, A. I. Mazun, I. B. Frolova, S. M. Gorlov and L. S. Nechaev, Steel in the USSR. 11 (1975) (beam blank sections averaged 550 cm.sup.2 in area, with dimensions of 245 .times.310 .times.130, the web length being shorter than the flange height); Ukrainian Metals Research Institute, USSR, described in V. T. Sladkoshteev, M. S. Gordienko, N. F. Gritsuk, R. V. Potanin and L. D. Kutsenko, Stal, 7 (1976) (beam blank sections averaged 520 cm.sup.2 in area, with dimensions of 415.times.284.times.50); and British Steel Corp., General Steels Division, Stoke-on-Trent, U.K. (beam blank sections averaged 790 cm.sup.2, with dimensions of 286.times.355.times.178 mm [111/2".times.14".times.7"], the web length being shorter than the flange height).
Other comments relating to shaped cross section casting and continuous casting devices for shaped cross section casting to produce, among other cross-sectional forms, beam blanks, appeared in various articles and papers, including G. S. Lucenti, Iron and Steel Engineer (July 1969); Y. Yagi, H. Fastert and H. Tokunaga, 1975 AISE Annual Convention (Cleveland, Ohio); K. Ushijima, Transactions ISIJ. 15 (1975); T. Saito, M. Kodama, and K. Komoda, Iro and Steel International, 48 (October 1975); and W. Puppe and H. Schenck, Stahl und Eisen 95, 25 (December 4, 1975).
Hartmann European Patent Application 0 297 258 (assigned to SMS Schloemann-Siemag AG), discloses a mold for the continuous casting of a "pre-profiles for beam rolling" (continuously cast beam blanks), which is used in combination with a submerged casting tube in the web portion of the mold. The mold is independently adjustable with respect to web height, web thickness and flange thickness, allowing variation of all three dimensions to produce a beam blank consisting of a web and two flanges. The Hartmann mold is also configured to comprise, in the web area, a widened arch-like or bulged metal inlet area, to afford ready introduction of the melt through a casting dip tube submerged under the bath surface, and to provide good distribution of the cast metal to the end areas of the blank. No relationship between web thickness and the width of the flange precursor portions arguably castable through use of that mold is disclosed by Hartmann, nor is there any disclosure or allusion to a maximum web and/or flange or flange precursor thickness in the virtually infinite number of products which that mold could be used to prepare.
DE-AC 2 218 408, noted by Hartmann, discloses a mold in which molten steel is fed within the web portion of the mold from an intermediate container through a submerged casting dip tube. That mold is adjustable to change the flange thickness, but not to vary either the web height or the web thickness.
Other special mold configurations were disclosed as necessary to control the stress and cracking problems which the known beam blanks encountered. Masui et al. U.S. Pat. No. 4,565,236, issued January 21, 1986, teaches the avoidance of cracks formed in the fillet parts of beam blanks, between the web and flange precursor portions, by the use of a mold cavity provided both with a taper at its web part in the casting direction, and variation in the curvature 1/R of the curved fillet parts of the mold cavity in the casting direction. The variation of the curvature is done in accordance with the amount of free shrinkage of the solidified shell of the beam blank strand (Abstract). Masui et al. state that their invention is particularly significant in the casting of beam blanks of large dimension or having a web height exceeding 775 MM (col. 10, 11. 53-65; FIG. 9, H=web height), and is the mechanism required to provide beam blanks with an inner web height (FIG. 9, W=inner web height) greater than 500 mm. No disclosure of attempting to avoid these problems by control of the maximum thickness of the various portions of the beam blank or the relationship of those portions to each other appears in Masui et al.
The continuous casting of shaped cross section beam blanks has the commercial advantage of enabling the production of a series of beam blanks from one or more heats of steel supplied to the process and apparatus, for as long a production run as the manufacturer chooses, without the need to first cast billet, reheat it and then subject that square stock to the processing necessary. In this manner, savings are achieved from the standpoint of producing a cast product that is closer to the final desired configuration than is achieved with either ingot casting or casting of a billet.
It is also known to produce beam blanks by continuously casting the metal in molten form into a continuous casting mold having what could be described as a "dog-bone"-shaped cross-section, a variation on the hour glass-type cross section. A particular example of the known practices for producing "dog-bone" shaped beam blanks by continuous casting is described in Lorento U.S. Pat. No. 4,805,685, issued Feb. 21, 1989. "Dog-bone" shaped beam blanks have been produced in commercial installations, with web thicknesses of at least four (4) inches and with flange or flange precursor portions of much greater size and thickness.
All of the aforenoted conventional practices and the beam blanks resulting therefrom have the disadvantage that the expanded end portions of the beam blank, the flange precursor portions, because of their increased cross-sectional area relative to the web portion of the beam blank, together with the thick web portion, require extensive hot rolling to achieve the final, required flange structure of the beam. This adds considerably to the complexity and overall cost of producing the beam, particularly in energy costs. In addition, high-cost heavy-duty hot rolling mills or millstands are required to achieve the necessary reductions of the expanded end portions of the beam blank, as well as cold rolling mill or millstand equipment for finishing operations (straightening and cutting to length), all of which comprise a tremendous required capital investment. The various continuously-cast shaped beam blanks known in the art must also be subjected to these substantial levels of hot working not just to achieve the final desired beam dimensions, but also to provide the necessary metallurgical structures and properties (including crystallization) of the metal required to be present in the finished structural member.
With respect to the BISRA laboratory work, for example, it was found that a hot working reduction of at least 6:1 was necessary to convert the as-cast shaped beam blank structure to attain final product dimension and to achieve the necessary metallurgical properties (H. S. Marr et al, supra). For a series of finished I beam sizes, the actual reduction was far higher, averaging between about 8:1 to about 10.5:1:
______________________________________ Rolled Beam Size Inch mm Area Reduction H .times. B H .times. B cm2 in Area ______________________________________ 14 .times. 63/4 356 .times. 171 64.5 10.4:1 16 .times. 7 406 .times. 178 76.1 8.8:1 16 .times. 7 406 .times. 178 68.4 9.8:1 18 .times. 71/2 457 .times. 191 85.1 7.9:1 ______________________________________
The Algoma Steel Corporation installation required an equivalent level of necessary further hot-working, with reduction ranging from about 6:1 to about 17.5:1:
______________________________________ Cast Beam Rolled Beam Size Blank inch mm Area Reduction Size H .times. B H .times. B cm2 in Area ______________________________________ 12 .times. 10 305 .times. 254 100.6 8.4:1 12 .times. 10 305 .times. 254 110.3 7.7:1 12 .times. 8 305 .times. 203 76.1 11.1:1 12 .times. 8 305 .times. 203 85.1 9.9:1 12 .times. 8 305 .times. 203 94.8 8.9:1 [173/4" .times. 12 .times. 61/2 305 .times. 165 51.0 16.6:1 12" .times. 4", 12 .times. 61/2 305 .times. 165 58.7 14.4:1 845 cm.sup.2 ] 12 .times. 61/2 305 .times. 165 68.4 12.4:1 14 .times. 8 356 .times. 203 81.3 10.4:1 14 .times. 8 356 .times. 203 90.9 9.3:1 14 .times. 8 356 .times. 203 100.6 8.4:1 14 .times. 63/4 356 .times. 171 56.8 14.9:1 14 .times. 63/4 356 .times. 171 64.5 13.1:1 14 .times. 63/4 356 .times. 171 72.2 11.7:1 18 .times. 71/2 457 .times. 191 76.1 11.5:1 18 .times. 71/2 457 .times. 191 85.1 10.3:1 18 .times. 71/2 457 .times. 191 94.8 9.2:1 18 .times. 71/2 457 .times. 191 104.5 8.4:1 [22" .times. 18 .times. 71/2 457 .times. 191 114.2 7.6:1 101/2" .times. 4", 16 .times. 7 406 .times. 178 60.6 14.4:1 873 cm.sup.2 ] 16 .times. 7 406 .times. 178 68.4 12.8:1 16 .times. 7 406 .times. 178 76.1 11.5:1 16 .times. 7 406 .times. 178 85.1 10.3:1 16 .times. 7 406 .times. 178 94.8 9.2:1 16 .times. 51/2 406 .times. 140 49.7 17.6:1 16 .times. 51/2 406 .times. 140 58.7 14.9:1 24 .times. 9 610 .times. 229 129.0 11.1:1 24 .times. 9 610 .times. 229 144.5 9.9:1 [301/2" .times. 24 .times. 9 610 .times. 229 159.3 9.0:1 14" .times. 4", 24 .times. 9 610 .times. 229 178.0 8.1:1 1434 cm.sup.2 ] 24 .times. 12 610 .times. 305 189.6 7.6:1 6.9:1 24 .times. 12 610 .times. 305 209.0 24 .times. 12 610 .times. 305 227.7 6.3:1 ______________________________________
Similarly, the Kawasaki Mizushima installation required hot-working reductions of about 9.5:1 to about 18:1, to achieve final product I beams with the desired size and requisite metallurgy:
______________________________________ Rolled Beam Size Area Reduction H .times. B (mm) cm2 in Area ______________________________________ 300 .times. 300 119.8 9.6:1 250 .times. 250 92.2 12.5:1 350 .times. 250 101.5 11.4:1 350 .times. 200 400 .times. 200 84.1 13.7:1 300 .times. 200 72.4 16.0:1 350 .times. 175 63.1 18.3:1 ______________________________________
While the known shaped continuous casting processes disclose a variety of beam blank sizes and configurations, there is no teaching or disclosure in the art of any intentional or recognized interrelationship between any of the parameters of the as-cast beam blank. Particularly lacking is any teaching or disclosure of limitation on the average thickness of the web portion of the blank, on the average thickness of the flange precursor portions of the blank, or any limitation or relationship between the average thickness of the flange precursor portions and the average thickness of the web, or any combination of a limitation on the average web thickness of the blank, and on the average flange precursor portion thickness of the blank, or further including a relationship between the average thickness of the flange precursor portions and the average thickness of the web.
The prior art continuously cast beam blanks all had at least a four (4) inch thick web portion, irrespective of whether the overall blank shape was rail-type in cross section, hour glass-type in cross section, or beam-type in cross section. These blanks had very thick flange precursor portions as well. The massiveness of the resulting blank was, in some measure, a primary reason for the substantial, costly hot-rolled reductions in cross-section and modifications in shape that the prior art mandated. It also presented an as-cast metallurgy that was unacceptable without substantial further hot-working, which, in most instances, could be effected before the required final dimensions of the structural member could be obtained. Preservation of the desired metallurgical properties through the further hot roll passes to complete the member proved difficult in most cases, impossible in many.
The existing continuously cast beam blanks and beam blank casting techniques were also limited by the known procedures needed to effect the casting operations.
The use of a submerged casting nozzle was taught by the prior art as necessary where commercial continuous casting speeds and commercial quality in the as-cast blank were required with thin section slab castings. Various submerged nozzle constructions, such as that disclosed in European Patent Application No. 0 336 158, were disclosed as useful in such casting procedures.
Due to the space relationships in the continuous casting mold, and the high casting speeds necessary and desired in commercial operations, there were difficulties in achieving a constant, controlled rate of solidification when thin sections were produced in thin slab casting operations. This often resulted in longitudinal cracks in casting certain steel grades, which presented severe quality and integrity problems. To avoid this problem, the use of a specially formulated casting powder was disclosed to be necessary. See H. J. Ehrenberg et al., Controlling of Thin Slabs At the Mannesmannrohren-Werke AG, MPT International, 12, 3/89, p.52.
The known techniques, then, mandated the use of both submerged nozzle pouring in the mold section and of casting powder, particularly where a thin section was required. Although not taught in the art, any attempt to use thin slab casting concepts in connection with beam blank casting would of necessity include submerged nozzle pouring and casting powder use.
Each of the known prior continuously cast beam blanks or pre-forms, and the techniques for producing them, suffered from a variety of serious shortcomings and problems. In all of the known prior continuously cast beam blanks, the web thickness substantially exceeded three (3) inches, usually exceeding four (4) inches. The "ears" portions (or flange precursor portions) of these blanks was massive in relation to said web thicknesses. During cooling and solidification of the metal during the continuous casting of these beam blanks in the manner known in the prior art, temperature gradients form in the liquid metal. These gradients promote the formation of a columnar structure. The beam blanks are often as a result characterized by a micro-structure having planes of weakness throughout the cross-section resulting in inferior metallurgical properties, particularly ductility and toughness.
Also, the amount of hot working, through use of conventional rolling techniques using known millstand-type equipment, is very substantial, averaging in excess of 15 passes, with up to 32 passes being necessary. The capital expenditure for the required rolling equipment is very substantial, and the time necessary and energy expended to make the high number of passes needed is not inconsequential. Achievement and preservation of desired metallurgy through the rolling regimen is complicated. Undesired and uncontrolled over-or under-elongation of the web portion of the blank is often experienced and difficult to accurately predict or control. Further, tearing of flange precursor/flange portions of the beam is a constant and substantial problem, as is buckling of the web portion. Restrictions on pouring points and technique are severe: open pouring had to be carried out into the mold zone corresponding to the approximate center of one of the massive "ear" portions of the known blank structures.
No teaching of any relationship between web or flange thickness in a cast beam blank and ease of the achievement of desired metallurgical properties in the beam blank or product has been advanced, nor has there been any disclosure relating web thickness to the thickness of the flange precursor portions of the beam blank in any manner, with or without control of the maximum web or flange thickness.
There was thus a need for an as-continuously cast beam blank and process for producing same, that:
1. Approximates the finished shape and configuration of the beam or other structural shape desired;
2. Minimizes the number of hot rolling passes or steps that must be undergone to reach the desired final size, which in turn would minimize the capital expenditure required to produce such blanks, and would markedly reduce the extreme energy costs which marked the prior art process;
3. Provides the desired metallurgical properties with the minimum number of rolling steps possible, and preserves those properties through any minimal additional rolling steps needed to reach desired final size, the number of steps required to obtain the desired metallurgical properties being substantially less than the number required with known beam blanks and processes;
4. Does not require the use of submerged pour techniques, and does not require the use of casting powder; and
5. Controls the relationship between web thickness and flange precursor thickness, to effect control over both required working and minimize tearing of flanges and undesired elongation and/or buckling of web portions and resulting distortion of the blank, as well as providing for rapid solidification in the mold with its accompanying metalurgical property benefits.
No available continuously cast beam blank, or process for producing same, provided the noted combination of advantages--minimal number of rolling passes to achieve both finished shape and desired metallurgy, with no undue web elongation or buckling or flange tearing; ability to use open pouring techniques and avoid mandatory use of submerged casting techniques, and/or casting powder, even where thin cross section webs are required; and improved, metallurgical characteristics which is carried into the finished beam and conserved by control over the number of hot rolling passes needed to reach final dimension and product configuration.